The present invention relates to highly heat resistant austenitic iron-nickel-chromium alloys which are resistant to neutron induced swelling as well as to corrosion by liquid sodium. Such alloys also contain small amounts of manganese, molybdenum, titanium, silicon, carbon, nitrogen, and boron.
The problem of neutron induced swelling has been known for more than a decade in nuclear structural materials used in fast nuclear reactors, particularly fast breeder reactors, and in the cladding and wrapper materials of the fuel elements used in these reactors. Initially, attempts were made to master this problem with the aid of structural measures. Later, stainless chromium-nickel-steel alloys of certain compositions were proposed which, under certain circumstances, were to be additionally subjected to subsequent thermal and/or mechanical treatments in order to reduce swelling of the material of the formation of cavities in the material.
The materials listed in German Industrial Standards DIN Nos. 1.4970 and 1.4981 have been used as cladding or wrapper materials, in connection with the German/Belgian/Netherlands fast breeder project. In other fast breeder projects the alloy employed was gradually the highly heat resistant austenitic steel known by the American Standard Term AISI 316. The British fast breeder project has selected the high nickel content austenitic material known by the trademark PE 16. The chemical compositions of these alloys are compiled in Table 1, below.
TABLE 1 ______________________________________ Compositions of prior art Fe--Cr--Ni steels or alloys. (percent by weight) High Nickel AISI Content Element DIN 1.4970 DIN 1.4981 316 (PE 16) ______________________________________ Cr 14.8 17.0 17.7 17.2 Ni 15.1 16.6 13.4 43.7 Mn 1.75 0.97 1.80 0.02 Mo 1.20 1.64 2.26 3.08 Ti 0.40 0.92 Si 0.40 0.58 0.36 0.10 C 0.10 0.06 0.057 0.07 N 0.02 0.02 0.001 0.011 B 0.005 0.0004 0.0005 0.001 Al 0.94 Zr 0.015 Nb 0.70 Cu 0.18 Fe Remainder Remainder Remainder Remainder ______________________________________
Attempts have also been made to optimize these steels with respect to swelling by suitable thermal or mechanical pretreatments, such as 20% cold working. The low nickel content commercial austenites, such as, AISI 316 and DIN 1.4970 exhibit a relatively high degree of swelling when in the solution heat treated state: approximately 6 to 10% at 40 dpa or 8.times.10.sup.22 neutrons per cm.sup.2 at 500.degree. C..+-.25.degree. C. With the use of cold working, this swelling can be reduced. In general the displacement damage during irradiation is given in displacements per (lattice) atom with the abbreviation dpa. The physical model underlying these calculations is the Norgett-Robinson-Torrens model. However, under the influence of radiation at higher process temperatures, those of at least 550.degree. C., there occurs accelerated recovery and/or recrystallisation of the cold working.
It is also possible to reduce swelling by utilizing high nickel content austenites such as PE 16, containing approximately 40% nickel. At comparable neutron doses and temperatures, swelling lies at about 1%. With this alloy, it is necessary to revert to hardening by means of the .gamma.'-phase precipitation mechanism, in order to attain sufficient strength. In this case, the precipitation of the .gamma.' phase consisting of Ni.sub.3 Al or Ni.sub.3 (AlTi) can be realized only by heat treatment at 700.degree. to 800.degree. C. The high nickel content austenites and nickel alloys are disadvantageous in their reduction of the breeding rate and an increase in wear by corrosion in liquid sodium.